Abstract: Species distribution patterns shaped by environmental gradients and their underlying topographical features have long been studied to understand processes of species adaptation and community assembly. However, it remains unclear whether slope-aspect and its associated environmental conditions promote differences in species distributions and functional traits, particularly among closely related lineages. To address this question, we randomly selected plots in north- and south-facing slopes at the same elevation and degree of slope, and recorded all oak trees (Quercus L.) in a semiarid region of central Mexico. For each plot, we characterized environmental conditions using high-resolution environmental data and measurements of plant water potential, and estimated the standing biomass using allometric equations. We also calculated community-weighted means of four leaf functional traits, and leaf deciduousness using normalized-difference vegetation index (NDVI). Our results showed that south-facing slopes experienced higher evaporative demand and greater soil water deficit, particularly during dry season. Oak species exhibited contrasting distribution patterns between slopes-aspects; Quercus laurina Bonpl., Q. rugosa Née and Q. scytophylla Liemb. were found at the more mesic north-facing slopes and Q. jonesii Trel., Q. deserticola Trel. and Q. eduardii Trel. were primarily found on the drier south-facing slopes. In addition, oak communities on south facing-slopes had larger leaf size and lower Huber value, traits associated with higher carbon acquisition capacity, and were more deciduous than those on north-facing slopes. Overall, these findings demonstrate that fine-scale topographic heterogeneity drives functional differentiation and spatial-niche partitioning, thereby promoting the local coexistence of oak species at a regional scale.

Key words: Mexican oaks, distribution patterns, Quercus, slope-aspect, leaf functional traits, environmental gradients